Navigation systems and methods for indicating and reducing line-of-sight errors

ABSTRACT

Navigation systems and methods used to track objects moving in space. One navigation system and method employs trackers having light emitters for indicating when line-of-sight between tracking elements on the trackers and sensors is obstructed or for other errors. Another navigation system and method instructs users on how to place a tracker with respect to an object to reduce line-of-sight issues during a surgical procedure.

RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 61/753,219, filed on Jan. 16, 2013, the entirecontents of which are hereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to navigation systems and methods forindicating and reducing line-of-sight errors when tracking one or moreobjects.

BACKGROUND

Navigation systems assist users in locating objects. For instance,navigation systems are used in industrial, aerospace, defense, andmedical applications. In the medical field, navigation systems assistsurgeons in placing surgical instruments relative to a patient'sanatomy. Surgeries in which navigation systems are used includeneurosurgery and orthopedic surgery. Typically, the instrument and theanatomy are tracked together with their relative movement shown on adisplay.

Navigation systems may employ light signals, sound waves, magneticfields, radio frequency signals, etc. in order to track the positionand/or orientation of objects. Often the navigation system includestracking devices attached to the object being tracked. A localizercooperates with tracking elements on the tracking devices to determine aposition of the tracking devices, and ultimately to determine a positionand/or orientation of the object. The navigation system monitorsmovement of the object via the tracking devices.

Many navigation systems rely on an unobstructed line-of-sight betweenthe tracking elements and sensors that receive tracking signals from thetracking elements. When the line-of-sight is obstructed, trackingsignals being transmitted from the tracking elements are not received bythe sensors. As a result, errors can occur. Typically, in thissituation, navigation is discontinued and error messages are conveyed tothe user until the line-of-sight returns or the system is reset. In themedical field, in many instances, the error messages are displayed on amonitor remote from the surgeon making it difficult for the surgeon tonotice and remedy the error in a timely manner. This can cause delays tosurgical procedures.

As a result, there is a need in the art for navigation systems andmethods that quickly identify line-of-sight issues so that they can beresolved without significant delay. There is also a need in the art fornavigation systems and methods that help to improve the line-of-sightand reduce possible errors associated with obstructions to theline-of-sight between tracking elements and sensors.

SUMMARY

In one embodiment, a navigation system is provided for tracking anobject. The navigation system comprises a tracking element fortransmitting a tracking signal. A sensor is provided to receive thetracking signal from the tracking element to determine a position of theobject. One of the tracking element and the sensor is attachable to theobject. An error indicator is also attachable to the object. The errorindicator indicates an error in the sensor receiving the tracking signalfrom the tracking element. The error indicator includes a first lightemitter and a second light emitter. A computing system determines if thesensor receives the tracking signal from the tracking element. Thecomputing system also generates an error signal if the sensor did notreceive the tracking signal and activates the first light emitter inresponse to generating the error signal. The computing system furtherdeactivates the second light emitter in response to generating the errorsignal.

A method is also provided for tracking an object. The method includesattaching one of a tracking element and a sensor to the object. Themethod also includes transmitting a tracking signal from the trackingelement and determining if the sensor received the tracking signal fromthe tracking element. An error signal is generated if the sensor did notreceive the tracking signal. A first light emitter is activated inresponse to the error signal and a second light emitter is deactivatedin response to the error signal.

One advantage of this navigation system and method is that the error isidentified directly on the object being tracked. As a result, the usercan better isolate signal transmission issues and remedy the issueswithout significant delay. Also, in some instances, by placing theindicator on the object being tracked, there is a greater likelihoodthat the user will notice the error as compared to an error messagedisplayed on a remote monitor.

In another embodiment a navigation system is provided to reduceline-of-sight issues while tracking an object. The navigation systemcomprises a tracking device. The tracking device includes a baseattachable to the object, a tracking element, and a connector couplingthe tracking element to the base to allow movement of the trackingelement in at least one degree of freedom relative to the base to placethe tracking element in a desired orientation. A computing systemdetermines if the tracking element is in the desired orientation andinstructs a user to move the tracking element relative to the base inthe at least one degree of freedom if the tracking element is not in thedesired orientation.

A method is also provided for setting up a navigation system to track anobject wherein the navigation system comprises a computing system and atracking device including a base, a tracking element, and a connector.The method comprises attaching the base to the object and determining ifthe tracking element is in a desired orientation. A user is instructedto move the tracking element relative to the base in at least one degreeof freedom if the tracking element is not in the desired orientation.

One advantage of this navigation system and method is to guide the userduring setup of the tracking devices so that the tracking devices arearranged in desired orientations. For instance, the desired orientationmay provide the tracking device with the necessary line-of-sight toother devices such as sensors of the navigation system.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 is a perspective view of a navigation system of the presentinvention being used in conjunction with a robotic manipulator;

FIG. 2 is a schematic view of the navigation system;

FIG. 3 is schematic view of the coordinates systems used in thenavigation system;

FIG. 4 is an exploded view of a tracking device;

FIG. 5 is an elevational view of the tracking device of FIG. 4;

FIG. 6 is a top view of the tracking device of FIG. 4;

FIG. 7 is a perspective view of a bone plate;

FIG. 8 is a top view of the bone plate;

FIG. 9 is a cross-sectional view of the bone plate taken generally alongthe line 9-9 in FIG. 8;

FIG. 10 is a blown-up view from FIG. 9;

FIG. 11 is a perspective view of a bone screw;

FIG. 12 is an elevational view of the bone screw;

FIG. 13 is a cross-sectional view of the bone screw taken down thecenter of the bone screw;

FIG. 14 is a blown-up view from FIG. 13;

FIG. 15 is an end view of the bone screw of FIG. 12;

FIG. 16 is an elevational view of an extension arm;

FIG. 17 is a cross-sectional view of the extension arm taken down thecenter of the extension arm;

FIG. 18 is a rear view of the extension arm;

FIG. 19 is a top view of the extension arm;

FIG. 20 is a blown-up view from FIG. 17;

FIG. 21 is an elevational view of the tracker assembly of FIG. 4 shownattached to bone;

FIG. 22 is a perspective view of a tracking head;

FIG. 23 is a perspective view of a connector;

FIG. 24 is a top view of the connector;

FIG. 25 is a cross-sectional view of the connector taken generally alongthe line 25-25 in FIG. 24;

FIG. 26 is a cross-sectional view of the connector taken generally alongthe line 26-26 in FIG. 24;

FIG. 27 is a perspective view of an alternative tracking head;

FIG. 28 is a perspective view of yet another alternative tracking head;

FIG. 29 is a perspective view of a light ring;

FIG. 30 is a schematic view of the navigation system;

FIG. 31 is an electrical schematic of two indicating LEDs;

FIG. 32 is an electrical schematic of four tracking LEDs;

FIGS. 33A and 33B are schematic views of a tracking head in differenttilt positions relative to gravity;

FIGS. 34A and 34B are schematic views of the tracking head in differentrotational positions;

FIG. 35 is a perspective view of a screw driver;

FIG. 36 is a cross-sectional view of the screw driver taken down thecenter of the screw driver;

FIG. 37 is a perspective view of a nose tube;

FIG. 38 is a cross-sectional view of the nose tube taken down the centerof the nose tube;

FIG. 39 is a cross-sectional view of the nose tube taken generally alongline 39-39 in FIG. 38;

FIG. 40 is a perspective view of a middle tube;

FIG. 41 is a cross-sectional view of the middle tube taken down thecenter of the middle tube;

FIG. 42 is a perspective view of a rear cap;

FIG. 43 is a cross-sectional view of the rear cap taken down the centerof the rear cap;

FIG. 44 is a perspective view of a hammer;

FIG. 45 is an elevational view of the hammer;

FIG. 46 is a cross-sectional view of the hammer taken generally alongthe line 46-46 of FIG. 45;

FIG. 47 is an end view of a trigger member;

FIG. 48 is a cross-sectional view of the trigger member taken down thecenter of the trigger member;

FIG. 49 is a cross-sectional view of a spring cap;

FIG. 50 is an elevational view of a driving rod; and

FIG. 51 is a cross-sectional view of the driving rod taken generallyalong the line 51-51 in FIG. 50.

DETAILED DESCRIPTION

Referring to FIG. 1 a navigation system 20 is illustrated. Thenavigation system 20 is shown in a surgical setting such as an operatingroom of a medical facility. The navigation system 20 is set up to trackmovement of various objects in the operating room. Such objects include,for example, a surgical instrument 22, a femur F of a patient, and atibia T of the patient. The navigation system 20 tracks these objectsfor purposes of displaying their relative positions and orientations tothe surgeon and, in some cases, for purposes of controlling orconstraining movement of the surgical instrument 22 relative to apredefined path or anatomical boundary.

The navigation system 20 includes a computer cart assembly 24 thathouses a navigation computer 26. A navigation interface is in operativecommunication with the navigation computer 26. The navigation interfaceincludes a first display 28 adapted to be situated outside of a sterilefield and a second display 29 adapted to be situated inside the sterilefield. The displays 28, 29 are adjustably mounted to the computer cartassembly 24. First and second input devices 30, 32 such as a mouse andkeyboard can be used to input information into the navigation computer26 or otherwise select/control certain aspects of the navigationcomputer 26. Other input devices are contemplated including a touchscreen (not shown) on displays 28, 29 or voice-activation.

A localizer 34 communicates with the navigation computer 26. In theembodiment shown, the localizer 34 is an optical localizer and includesa camera unit 36 (also referred to as a sensing device). The camera unit36 has an outer casing 38 that houses one or more optical positionsensors 40. In some embodiments at least two optical sensors 40 areemployed, preferably three. The optical sensors 40 may be three separatecharge-coupled devices (CCD). In one embodiment three, one-dimensionalCCDs are employed. It should be appreciated that in other embodiments,separate camera units, each with a separate CCD, or two or more CCDs,could also be arranged around the operating room. The CCDs detectinfrared (IR) signals.

Camera unit 36 is mounted on an adjustable arm to position the opticalsensors 40 with a field of view of the below discussed trackers that,ideally, is free from obstructions.

The camera unit 36 includes a camera controller 42 in communication withthe optical sensors 40 to receive signals from the optical sensors 40.The camera controller 42 communicates with the navigation computer 26through either a wired or wireless connection (not shown). One suchconnection may be an IEEE 1394 interface, which is a serial businterface standard for high-speed communications and isochronousreal-time data transfer. The connection could also use a companyspecific protocol. In other embodiments, the optical sensors 40communicate directly with the navigation computer 26.

Position and orientation signals and/or data are transmitted to thenavigation computer 26 for purposes of tracking the objects. Thecomputer cart assembly 24, display 28, and camera unit 36 may be likethose described in U.S. Pat. No. 7,725,162 to Malackowski, et al. issuedon May 25, 2010, entitled “Surgery System”, hereby incorporated byreference.

The navigation computer 26 can be a personal computer or laptopcomputer. Navigation computer 26 has the displays 28, 29, centralprocessing unit (CPU) and/or other processors, memory (not shown), andstorage (not shown). The navigation computer 26 is loaded with softwareas described below. The software converts the signals received from thecamera unit 36 into data representative of the position and orientationof the objects being tracked.

Navigation system 20 includes a plurality of tracking devices 44, 46,48, also referred to herein as trackers. In the illustrated embodiment,one tracker 44 is firmly affixed to the femur F of the patient andanother tracker 46 is firmly affixed to the tibia T of the patient.Trackers 44, 46 are firmly affixed to sections of bone. Trackers 44, 46may be attached to the femur F and tibia T in the manner shown in U.S.Pat. No. 7,725,162, hereby incorporated by reference. Other methods ofattachment are described further below. In additional embodiments, atracker (not shown) is attached to the patella to track a position andorientation of the patella. In yet further embodiments, the trackers 44,46 could be mounted to other tissue types or parts of the anatomy.

An instrument tracker 48 is firmly attached to the surgical instrument22. The instrument tracker 48 may be integrated into the surgicalinstrument 22 during manufacture or may be separately mounted to thesurgical instrument 22 in preparation for the surgical procedures. Theworking end of the surgical instrument 22, which is being tracked, maybe a rotating bur, electrical ablation device, or the like.

The trackers 44, 46, 48 can be battery powered with an internal batteryor may have leads to receive power through the navigation computer 26,which, like the camera unit 36, preferably receives external power.

In the embodiment shown, the surgical instrument 22 is an end effectorof a surgical manipulator. Such an arrangement is shown in U.S. patentapplication Ser. No. 13/958,070, filed Aug. 2, 2013, entitled, “SurgicalManipulator Capable of Controlling a Surgical Instrument in MultipleModes”, the disclosure of which is hereby incorporated by reference.

In other embodiments, the surgical instrument 22 may be manuallypositioned by only the hand of the user, without the aid of any cuttingguide, jib, or other constraining mechanism such as a manipulator orrobot. Such a surgical instrument is described in U.S. patentapplication Ser. No. 13/600,888, filed Aug. 31, 2012, entitled,“Surgical Instrument Including Housing, a Cutting Accessory that Extendsfrom the Housing and Actuators that Establish the Position of theCutting Accessory Relative to the Housing”, the disclosure of which ishereby incorporated by reference.

The optical sensors 40 of the localizer 34 receive light signals fromthe trackers 44, 46, 48. In the illustrated embodiment, the trackers 44,46, 48 are active trackers. In this embodiment, each tracker 44, 46, 48has at least three active tracking elements or markers 50 fortransmitting light signals to the optical sensors 40. The active markers50 can be, for example, light emitting diodes (LEDs) 50 transmittinglight, such as infrared light. The optical sensors 40 preferably havesampling rates of 100 Hz or more, more preferably 300 Hz or more, andmost preferably 500 Hz or more. In some embodiments, the optical sensors40 have sampling rates of 1000 Hz. The sampling rate is the rate atwhich the optical sensors 40 receive light signals from sequentiallyfired LEDs 50. In some embodiments, the light signals from the LEDs 50are fired at different rates for each tracker 44, 46, 48.

Referring to FIG. 2, each of the LEDs 50 are connected to a trackercontroller 62 located in a housing (not shown) of the associated tracker44, 46, 48 that transmits/receives data to/from the navigation computer26. In one embodiment, the tracker controllers 62 transmit data on theorder of several Megabytes/second through wired connections with thenavigation computer 26. In other embodiments, a wireless connection maybe used. In these embodiments, the navigation computer 26 has atransceiver (not shown) to receive the data from the tracker controller62.

In other embodiments, the trackers 44, 46, 48 may have passive markers(not shown), such as reflectors that reflect light emitted from thecamera unit 36. The reflected light is then received by the opticalsensors 40. Active and passive marker arrangements are well known in theart.

Each of the trackers 44, 46, 48 also includes a 3-dimensional gyroscopesensor 60 that measures angular velocities of the trackers 44, 46, 48.As is well known to those skilled in the art, the gyroscope sensors 60output readings indicative of the angular velocities relative to x, y,and z axes of a gyroscope coordinate system. These readings aremultiplied by a conversion constant defined by the manufacturer toobtain measurements in degrees/second with respect to each of the x, y,and z axes of the gyroscope coordinate system. These measurements canthen be converted to an angular velocity vector defined inradians/second.

The angular velocities measured by the gyroscope sensors 60 provideadditional non-optically based kinematic data for the navigation system20 with which to track the trackers 44, 46, 48. The gyroscope sensors 60may be oriented along the axis of each coordinate system of the trackers44, 46, 48. In other embodiments, each gyroscope coordinate system istransformed to its tracker coordinate system such that the gyroscopedata reflects the angular velocities with respect to the x, y, and zaxes of the coordinate systems of the trackers 44, 46, 48.

Each of the trackers 44, 46, 48 also includes a 3-axis accelerometer 70that measures acceleration along each of x, y, and z axes of anaccelerometer coordinate system. The accelerometers 70 provideadditional non-optically based data for the navigation system 20 withwhich to track the trackers 44, 46, 48.

The accelerometers 70 may be oriented along the axis of each coordinatesystem of the trackers 44, 46, 48. In other embodiments, eachaccelerometer coordinate system is transformed to its tracker coordinatesystem such that the accelerometer data reflects the accelerations withrespect to the x, y, and z axes of the coordinate systems of thetrackers 44, 46, 48.

Each of the gyroscope sensors 60 and accelerometers 70 communicate withthe tracker controller 62 located in the housing of the associatedtracker that transmits/receives data to/from the navigation computer 26.The data can be received either through a wired or wireless connection.

The navigation computer 26 includes a navigation processor 52. Thecamera unit 36 receives optical signals from the LEDs 50 of the trackers44, 46, 48 and outputs to the processor 52 signals and/or data relatingto the position of the LEDs 50 of the trackers 44, 46, 48 relative tothe localizer 34. The gyroscope sensors 60 transmit non-optical signalsto the processor 52 relating to the 3-dimensional angular velocitiesmeasured by the gyroscope sensors 60. Based on the received optical andnon-optical signals, navigation processor 52 generates data indicatingthe relative positions and orientations of the trackers 44, 46, 48relative to the localizer 34.

It should be understood that the navigation processor 52 could includeone or more processors to control operation of the navigation computer26. The processors can be any type of microprocessor or multi-processorsystem. The term processor is not intended to be limited to a singleprocessor.

Prior to the start of the surgical procedure, additional data are loadedinto the navigation processor 52. Based on the position and orientationof the trackers 44, 46, 48 and the previously loaded data, navigationprocessor 52 determines the position of the working end of the surgicalinstrument 22 and the orientation of the surgical instrument 22 relativeto the tissue against which the working end is to be applied. In someembodiments, navigation processor 52 forwards these data to amanipulator controller 54. The manipulator controller 54 can then usethe data to control a robotic manipulator 56 as described in U.S. patentapplication Ser. No. 13/958,070, filed Aug. 2, 2013, entitled, “SurgicalManipulator Capable of Controlling a Surgical Instrument in MultipleModes”, the disclosure of which is hereby incorporated by reference.

The navigation processor 52 also generates image signals that indicatethe relative position of the surgical instrument working end to thesurgical site. These image signals are applied to the displays 28, 29.Displays 28, 29, based on these signals, generate images that allow thesurgeon and staff to view the relative position of the surgicalinstrument working end to the surgical site. The displays, 28, 29, asdiscussed above, may include a touch screen or other input/output devicethat allows entry of commands.

Referring to FIG. 3, tracking of objects is generally conducted withreference to a localizer coordinate system LCLZ. The localizercoordinate system has an origin and an orientation (a set of x, y, and zaxes).

Each tracker 44, 46, 48 and object being tracked also has its owncoordinate system separate from localizer coordinate system LCLZ.Components of the navigation system 20 that have their own coordinatesystems are the bone trackers 44, 46 and the instrument tracker 48.These coordinate systems are represented as, respectively, bone trackercoordinate systems BTRK1, BTRK2, and instrument tracker coordinatesystem TLTR.

Navigation system 20 monitors the positions of the femur F and tibia Tof the patient by monitoring the position of bone trackers 44, 46 firmlyattached to bone. Femur coordinate system is FBONE and tibia coordinatesystem is TBONE, which are the coordinate systems of the bones to whichthe bone trackers 44, 46 are firmly attached.

Prior to the start of the procedure, pre-operative images of the femur Fand tibia T are generated (or of other tissues in other embodiments).These images may be based on magnetic resonance imaging (MRI) scans,radiological scans or computed tomography (CT) scans of the patient'sanatomy. These images are mapped to the femur coordinate system FBONEand tibia coordinate system TBONE using well known methods in the art.In one embodiment, a pointer instrument P, such as disclosed in U.S.Pat. No. 7,725,162 to Malackowski, et al., hereby incorporated byreference, having its own tracker PT (see FIG. 2), may be used to mapthe femur coordinate system FBONE and tibia coordinate system TBONE tothe pre-operative images. These images are fixed in the femur coordinatesystem FBONE and tibia coordinate system TBONE.

During the initial phase of the procedure, the bone trackers 44, 46 arefirmly affixed to the bones of the patient. The pose (position andorientation) of coordinate systems FBONE and TBONE are mapped tocoordinate systems BTRK1 and BTRK2, respectively. Given the fixedrelationship between the bones and their bone trackers 44, 46, the poseof coordinate systems FBONE and TBONE remain fixed relative tocoordinate systems BTRK1 and BTRK2, respectively, throughout theprocedure. The pose-describing data are stored in memory integral withboth manipulator controller 54 and navigation processor 52.

The working end of the surgical instrument 22 (also referred to asenergy applicator distal end) has its own coordinate system EAPP. Theorigin of the coordinate system EAPP may represent a centroid of asurgical cutting bur, for example. The pose of coordinate system EAPP isfixed to the pose of instrument tracker coordinate system TLTR beforethe procedure begins. Accordingly, the poses of these coordinate systemsEAPP, TLTR relative to each other are determined. The pose-describingdata are stored in memory integral with both manipulator controller 54and navigation processor 52.

Referring to FIG. 2, a localization engine 100 is a software module thatcan be considered part of the navigation system 20. Components of thelocalization engine 100 run on navigation processor 52. In someversions, the localization engine 100 may run on the manipulatorcontroller 54.

Localization engine 100 receives as inputs the optically-based signalsfrom the camera controller 42 and the non-optically based signals fromthe tracker controller 62. Based on these signals, localization engine100 determines the pose of the bone tracker coordinate systems BTRK1 andBTRK2 in the localizer coordinate system LCLZ. Based on the same signalsreceived for the instrument tracker 48, the localization engine 100determines the pose of the instrument tracker coordinate system TLTR inthe localizer coordinate system LCLZ.

The localization engine 100 forwards the signals representative of theposes of trackers 44, 46, 48 to a coordinate transformer 102. Coordinatetransformer 102 is a navigation system software module that runs onnavigation processor 52. Coordinate transformer 102 references the datathat defines the relationship between the pre-operative images of thepatient and the patient trackers 44, 46. Coordinate transformer 102 alsostores the data indicating the pose of the working end of the surgicalinstrument 22 relative to the instrument tracker 48.

During the procedure, the coordinate transformer 102 receives the dataindicating the relative poses of the trackers 44, 46, 48 to thelocalizer 34. Based on these data and the previously loaded data, thecoordinate transformer 102 generates data indicating the relativeposition and orientation of both the coordinate system EAPP, and thebone coordinate systems, FBONE and TBONE to the localizer coordinatesystem LCLZ.

As a result, coordinate transformer 102 generates data indicating theposition and orientation of the working end of the surgical instrument22 relative to the tissue (e.g., bone) against which the instrumentworking end is applied. Image signals representative of these data areforwarded to displays 28, 29 enabling the surgeon and staff to view thisinformation. In certain embodiments, other signals representative ofthese data can be forwarded to the manipulator controller 54 to controlthe manipulator 56 and corresponding movement of the surgical instrument22.

Steps for determining the pose of each of the tracker coordinate systemsBTRK1, BTRK2, TLTR in the localizer coordinate system LCLZ and systemsand methods for determining the pose of the trackers 44, 46, 48 and thecorresponding poses of the surgical instrument 22 with respect to thefemur F and tibia T are described in greater detail in U.S. patentapplication Ser. No. 14/035,207, filed Sep. 24, 2013, entitled“Navigation System Including Optical and Non-Optical Sensors”, thedisclosure of which is hereby incorporated by reference.

In some embodiments, only one LED 50 can be read by the optical sensors40 at a time. The camera controller 42, through one or more infrared orRF transceivers (on camera unit 36 and trackers 44, 46, 48), or througha wired connection, may control the firing of the LEDs 50, as describedin U.S. Pat. No. 7,725,162 to Malackowski, et al., hereby incorporatedby reference. Alternatively, the trackers 44, 46, 48 may be activatedlocally (such as by a switch on trackers 44, 46, 48) which then firesits LEDs 50 sequentially once activated, without instruction from thecamera controller 42.

One embodiment of trackers 44, 46 is shown in FIGS. 4-6. Trackers 44, 46are configured to be attached to bone and fixed in position relative tothe bone. As a result, movement of the bone results in corresponding andlike movement of the trackers 44, 46. In some versions of the navigationsystem 20, both trackers 44, 46 comprise the same or substantially thesame components. In other versions, the trackers 44, 46 differ in one ormore components. For simplicity, only tracker 44 will be describedbelow, but it is understood that tracker 46 may be the same orsubstantially the same as tracker 44.

Referring to FIGS. 4-6, tracker 44 includes a base for attaching to thepatient's anatomy. The base can be attached directly to the anatomybeing tracked or through other tissue. In the embodiment shown, the baseis a bone plate 200 for attaching to the patient's bone—for instance thefemur F. A plurality of fasteners secures the bone plate 200 in place.In one embodiment, the fasteners are bone screws 202.

The bone plate 200 includes a plurality of protrusions for engaging thebone. In the embodiment shown, the protrusions are spikes 204. Once thebone plate 200 is secured in place with one or more bone screws, thespikes 204 prevent rotation of the bone plate 200 relative to the bone.

An extension arm 206 is mounted to the bone plate 200. The extension arm206 has a base plate 208 that is secured to the bone plate 200 with oneof the bone screws 202. The extension arm 206 extends arcuately in aC-shape from the base plate 208 to a mounting end 210.

A tracking head 212 is coupled to the mounting end 210 of the extensionarm 206. The tracking head 212 includes tracking elements. The trackingelements, in the embodiment shown and described, are the LEDs 50,gyroscope sensor 60 (not shown), and accelerometer 70 (not shown). Thesetracking elements operate as previously described. In furtherembodiments, the tracking head 212 may include other types of trackingelements such as radio frequency receivers and/or transmitters, magneticfield sensors and/or generators, passive reflector balls, ultrasonictransmitters and/or receivers, or the like.

A connector assembly 214 couples the tracking head 212 to the extensionarm 206. The connector assembly 214 supports the tracking head 212 formovement in two degree of freedom. In the embodiment shown, the trackinghead 212 is rotatably and tiltably mounted to the mounting end 210 ofthe support arm 206 via the connector assembly 214.

Referring to FIGS. 7-10, the bone plate 200 is shown in greater detail.The bone plate 200 is generally triangular and concave between eachspike 204 (see FIG. 9). This concavity conforms to or otherwiseaccommodates the shape of bone or other tissue to which the bone plate200 is to be secured.

The bone plate 200 has top and bottom surfaces 216, 218 with three sidesurfaces 220, 222, 224. The side surfaces 220, 222, 224 extend betweenthe top and bottom surfaces 216, 218. The concavity of the bone plate200 can be given by a radius of curvature R1 of the top surface 216 offrom about 5 millimeters to about 50 millimeters and a radius ofcurvature R2 of the bottom surface 218 of from about 5 millimeters toabout 50 millimeters (see FIG. 9). In other embodiments, the radius ofcurvature R1 is from about 15 millimeters to about 35 millimeters andthe radius of curvature R2 is from about 15 millimeters to about 35millimeters.

Three spikes 204 are formed as integral extensions of surfaces 218, 220,222, 224. Each spike 204 has a sharp tip 226 (see FIG. 10) that isformed near the intersection of the bottom surface 218 and two adjacentside surfaces 220, 222, 224. The bottom surface 218 extends arcuatelyalong each spike 204 to the sharp tip 226 to form a gradual taper to thesharp tip 226. The bottom surface 218 is generally concavely shapedbetween sharp tips 226.

The sharp tips 226 are formed to cut through soft tissue, such as theperiosteum, and pierce into bone when the bone plate 200 is secured tobone. When one or more of the sharp tips 226 pierce into bone, they, inconjunction with one or more of the bone screws 202, prevent movement ofthe bone plate 200 relative to the bone.

The sharp tips 226, when engaged in bone, also support the bone plate200 to provide a space beneath the bone plate 200 and above the surfaceof the bone. In some cases, tissue such as muscle, ligaments, and thelike may be present on top of the bone to which the bone plate 200 is tobe secured. This tissue can be accommodated in this space withoutaffecting the engagement of the sharp tips 226 in the bone.

Three openings 228 are defined through the bone plate 200 to receive thebone screws 202. These three openings 228 have the cross-sectionalconfiguration shown in FIG. 10. One embodiment of this configuration isshown in U.S. Pat. No. 6,322,562 to Wolter, hereby incorporated byreference, in order to receive threaded heads 205 of the bone screws 202shown in FIGS. 11-15.

Each of the openings 228 are defined about an axis A. Each opening 228comprises a generally cylindrical throughbore 230 defined by innersurface 234. The throughbore 230 is centered about the axis A.

An integral flange 232 is located in the throughbore 230 and directedradially inward toward axis A. The flange 232 is spaced from the top andbottom surfaces 216, 218 of the bone plate 200. This flange 232 isdisposed annularly about axis A and generally perpendicular to axis A.The flange 232 tapers in cross-section from the inner surface 234 to anend surface 236. The end surface 236 defines an opening (not numbered)that is cylindrical in shape. The taper of the flange 232 issymmetrically formed by upper and lower surfaces 240, 242. The uppersurface 240 extends at an acute angle α from end surface 236 to innersurface 234. The lower surface 242 extends at the same acute angle αfrom the end surface 236 to the inner surface 234, but in the oppositedirection.

Referring back to FIG. 7, a recess 244 is defined in the top surface 216of the bone plate 200. The recess 244 is generally rectangular in shapefor mating reception of the base plate 208 of the extension arm 206. Thebase plate 208 is sized so that once located in the recess 244 theextension arm 206 is substantially prevented from rotation relative tothe bone plate 200.

A central opening 246 is located in the recess 244 and is definedthrough the bone plate 200. The central opening 246 receives a bonescrew 202 similar to the openings 228, but has a different cross-sectionthan openings 228. The central opening 246 is a generally cylindricalthroughbore of single diameter that is substantially perpendicular tothe bone plate 200 at that location.

An axis C defines a center of the throughbore 246, as shown in FIG. 9.The openings 228 are spaced equidistantly from the axis C. Additionally,the openings 228 are spaced equally circumferentially about an imaginarycircle defined through axes A of each of the openings 228 (see FIG. 8).

Referring to FIG. 10, a normal directional vector V to the bone plate200 is defined downwardly along axis C toward the patient's anatomy whenmounted. Inclined directional vectors I, arranged at acute angle β tothe normal directional vector V, are defined downwardly along axes A. Inone embodiment, these inclined directional vectors I are generallydirected toward the normal directional vector V and cross the normaldirectional vector V at the same point INT along axis C. Thisorientation assists with preventing pull-out of the bone screws 202 fromforces acting on the bone plate 200.

The bone plate 200 may be formed of stainless steel, cobalt base alloys,bioceramics, titanium alloys, titanium, or other biocompatiblematerials.

Bone screws 202 are shown in FIGS. 11-15. In one embodiment, the bonescrews are similar to those shown in U.S. Pat. No. 6,322,562,incorporated by reference herein. Each of the bone screws 202 has selftapping threads 203 for engaging bone. The bone screws 202 also have thethreaded heads 205 for engaging the flanges 232 of the openings 228. Thebone screws 202 may be of different sizes depending on the tissue towhich they are being mounted. For instance, if the bone plate 200 isconfigured for mounting to a patella, smaller bone screws may beutilized. If the bone plate 200 is being mounted to the tibia, which istypically harder than other bones of the body, smaller bone screws maybe used. For soft bone, longer bone screws that implant deeper into thebone may be desired.

FIGS. 16-20 show the extension arm 206 in greater detail. The base plate208 of the extension arm 206 defines a plate opening 252 that is thesame in shape and size to the openings 228. The plate opening 252 hasthe same features as openings 228 and will not be described further.Plate opening 252 receives a central fastener such as bone screw 202 inthe same manner as openings 228. In the embodiment shown, the base plate208 is secured to the bone plate 200 by virtue of compression of thebase plate 208 against the bone plate 200 when the bone screw 202 isfastened to bone through the plate opening 252 and central opening 246.

An arcuate segment 254 extends from the base plate 208 to the mountingend 210. A rib 256 is disposed partially on the base plate 208, extendsalong the arcuate segment 254, and ends at the mounting end 210. The rib256 provides additional rigidity to the extension arm 206 to preventbending, buckling, twisting, or other deformation of the extension arm206.

A mounting surface 258 is located at the mounting end 210 of theextension arm 206. The mounting surface 258 is configured to support theconnector assembly 214 and tracking head 212. The mounting surface 258is generally planar. A threaded opening 260 is defined through themounting end 210 for receiving a threaded adjustment fastener 261 (seeFIG. 4).

The extension arm 206 interconnects the bone plate 200 and the trackinghead 212. The extension arm 206 spaces the tracking elements (such asLEDs 50) of the tracking head 212 from the bone plate 200. The trackingelements are spaced in this manner to extend above the anatomy therebyimproving line-of-sight potential between the tracking elements and theoptical sensors 40 of camera unit 36.

Referring to FIG. 21, the extension arm 206 is generally C-shaped todefine a tissue receiving area 262 between the tracker head 212 and thebone plate 200. The tissue receiving area 262 is configured to receivesoft tissue such as skin, fat, muscle, etc. above the bone plate 200when the bone plate 200 is mounted to the bone.

The tissue receiving area 262 enables the user to retract soft tissueaway from bone, mount the bone plate 200 directly to the bone, and thenrelease the soft tissue back to a position above the bone plate 200.Accordingly, the soft tissue is not required to be continually retractedduring the entire surgical procedure. FIG. 21 shows layers of skin, fat,muscle, and fascia being located in the tissue receiving area 262 whilethe bone plate 200 is mounted to the femur F. In particular, the sharptips 226 penetrate through the periosteum into the hard cortical bone ofthe femur F.

The bone plate 200 is firmly mounted in bone unicortically—meaning thebone screws 202 only penetrate the cortical bone layer once, from theoutside.

The extension arm 206 may be formed of stainless steel, cobalt basealloys, bioceramics, titanium alloys, titanium, or other biocompatiblematerials.

Referring to FIG. 22, the tracking head 212 includes the plurality ofLEDs 50, gyroscope sensor 60 (not shown), accelerometer 70 (not shown),and a transceiver (not shown) for receiving and transmitting signals toand from the camera unit 36 and/or navigation computer 26. The trackinghead 212 may also be connected to the navigation computer 26 via a wiredconnection as previously described.

The tracking head 212 includes a first hinge member 264 for mounting tothe connector assembly 214. The first hinge member 264 defines anon-threaded bore 268.

The connector assembly 214 is shown in FIGS. 4 and 23-26. The connectorassembly 214 includes a connector 270 for interconnecting the trackinghead 212 and the extension arm 206. The connector 270 includes a pair ofsecond hinge members 278. A space 280 is defined between the secondhinge members 278 for receiving the first hinge member 264. One of thesecond hinge members 278 has a non-threaded bore 282, while the otherhas a threaded bore 284. A threaded adjustment fastener 286 (see FIG. 4)passes through the non-threaded bore 282 into the threaded bore 284.

When tightening the adjustment fastener 286 the second hinge members 278are drawn together to compress against the first hinge member 264. Thisprevents movement of the first hinge member 264 relative to the secondhinge members 278. When the adjustment fastener 286 is loosened, thesecond hinge members 278 relax to a non-compressed position in which thefirst hinge member 264 is freely movable in the space 280.

The tracking head 212 can be tilted relative to the bone plate 200 viathe hinge created by the hinge members 264, 278. Tilting occurs in onedegree of freedom about pivot axis P (see FIG. 5). Pivot axis P isdefined centrally through the bores 268, 282, 284.

The connector 270 also has a rotational base 290. The rotational base290 is integral with the second hinge members 278. The rotational base290 has a flat bottom (not numbered) for mating with the mountingsurface 258.

The rotational base 290 defines an opening 292. The opening 292 isshaped to receive a frusto-conical head (not separately numbered) of theadjustment fastener 261 (see FIG. 4). The opening 292 also has acylindrical bore 294. The bore 294 is shaped so that a threaded shaft(not separately numbered) of the adjustment fastener 261 passestherethrough into the threaded opening 260 in the mounting end 210 ofthe extension arm 206.

When tightening the adjustment fastener 261 the rotational base 290 isdrawn against the mounting surface 258. Friction between the bottom ofthe rotational base 290 and the mounting surface 258 prevents rotationalmovement of the connector 270 relative to the mounting surface 258. Whenthe adjustment fastener 261 is loosened, the connector 270 can berotated freely relative to the mounting surface 258. Thus, the trackinghead 212 can be rotated relative to the bone plate 200. Rotation occursin one degree of freedom about rotational axis R (see FIGS. 5 and 6).Rotational axis R is defined centrally through bore 294 and threadedopening 260.

Some of the tracking elements, such as the LEDs 50, rely online-of-sight with the optical sensors 40 to transmit tracking signalsto the optical sensors 40. As a result, these tracking elements are alsoreferred to as line-of-sight tracking elements. These tracking elementsmust be within the field of view of the camera unit 36 and not beblocked from transmitting tracking signals to the camera unit 36. Whenthe signal path of one or more tracking elements is obstructed, an errormessage, in certain situations, may be generated.

The optical sensors 40 of the navigation system 20 are configured toreceive signals from the LEDs 50. The navigation system 20 controlsactivation of the LEDs 50, as previously described, so that thenavigation system 20 can anticipate when a signal should be received.When an anticipated signal from an LED 50 is not received onepossibility is that the signal path is obstructed and the signal isblocked from being sent to the camera unit 36. Another possibility isthat the LED 50 is not functioning properly.

The navigation computer 26 determines that there is an error if any oneof the optical sensors 40 fails to receive a signal from an LED 50, eventhough other sensors 40 still receive the signal. In other embodiments,navigation computer 26 determines that there is an error if none of theoptical sensors 40 receive the signal. In either case, when thenavigation system 20 determines that there is an error based on thefailure of one or more sensors 40 to receive signals from one or moreLEDs 50, an error signal is generated by the navigation computer 26. Anerror message then appears on displays 28, 29. The navigation computer26 also transmits an error signal to the tracker controller 62.

An error indicator 300 is located on tracking head 312 in the embodimentshown in FIG. 27. The tracker controller 62 activates the indicator 300so that the user is aware of the error, e.g., that the signal from oneor more of the LEDs 50 is blocked. Since the navigation computer 26 candetermine which specific LED or LEDs 50 has failed to successfullytransmit a signal to the optical sensor or sensors 40, in someembodiments each LED 50 on the tracker 40 may have a separate indicatorso that the user knows specifically which of the LEDs 50 are blocked.

The indicator 300 includes indicating light emitters such as indicatinglight emitting diodes (LEDs) 302. The indicating LEDs 302 emit a firstcolored light when the tracker controller 62 receives the error signalfrom the navigation computer 26, such as a red, yellow, or orangecolored light. The indicating LEDs 302 emit a second colored light whenno error signal is received or when an all clear signal is received fromthe navigation computer 26 after the error is cleared, such as a greenor blue colored light. This indicates that the line-of-sight is notbroken and is being maintained between at least a required number of theLEDs 50 and the optical sensor or sensors 40. When an error is againdetected the light changes from the second colored light to the firstcolored light. It should be appreciated that the indicating LEDs 302 mayinclude separate indicating LEDs that are alternately activated based onthe error status—one or more first colored indicating LEDs for error andone or more second colored indicating LEDs for no error. In otherembodiments, the indicator 300 may include an LED or LCD display witherror message and/or audible alerts when there is an error.

Tracking head 312 has a body 306 supporting the LEDs 302. The body 306defines openings (not numbered) covered by transparent windows 310. TheLEDs 302 are located inside the body 306 behind the windows 310 so thatlight from the LEDs 302 can be emitted through the windows 310 so thatthe light is visible to the user. The windows 310 may also includelenses (not shown) to provide desired lumination characteristics for theLEDs 302.

In another embodiment shown in FIGS. 28 and 29, tracking head 412supports first indicator LEDs 402 that emit orange colored light andsecond indicator LEDs 404 that emit green colored light. The trackinghead 412 includes a top 414 supporting the LEDs 50. Sapphire domes 418cover the LEDs 50. The tracking head 412 further includes a bottom 420.

A light ring 430 is captured between the top 414 and bottom 420. Theindicator LEDs 402, 404 are located inside the tracking head 412 withinthe light ring 430, as schematically shown in FIG. 29. The light ring430 is preferably formed of white alumina material and has a rectangularring shape. The light ring 430 illuminates either orange or green,depending on which of the indicator LEDs 402, 404 are activated. Theindicator LEDs 402, 404 are in electronic communication with the trackercontroller 62, as shown in FIG. 30. Weld rings 416 are located betweenthe light ring 430 and the bottom 420 to facilitate assembly.

The light ring 430 is illuminated with the orange colored light from thefirst indicator LEDs 402 when the tracker controller 62 receives theerror signal from the navigation computer 26. The light ring 430 isilluminated with the green colored light from the second indicator LEDs404 when no error signal is received or when an all clear signal isreceived from the navigation computer 26 after the error is cleared.When an error is again detected the light ring 430 changes from beingilluminated green to being illuminated orange. The indicator LEDs 402,404 are alternately activated based on the error status—one or morefirst indicator LEDs 402 are activated for error conditions and one ormore second indicator LEDs 404 are activated when no error conditionsexist.

Alternating activation of the indicator LEDs 402, 404 are carried outusing the circuit shown in FIG. 31. FIG. 31 shows an electricalschematic of the tracker controller 62 and the indicator LEDs 402, 404.The electrical schematic shows the electrical components that facilitatealternating activation/deactivation of the indicator LEDs 402, 404.

Each of the indicator LEDs 402, 404 includes an anode 432 and a cathode434. In FIG. 31, the anode 432 of each of the indicator LEDs 402, 404connects to a first voltage reference 436 and the cathode 434 of each ofthe indicator LEDs 402, 404 connects to a second voltage reference 438.In one embodiment, the first voltage reference 436 is +5 VDC and thesecond voltage reference 438 is signal ground. The anode 432 of each ofthe indicator LEDs 402, 404 may connect to the first voltage reference438 separately, or in combination, as shown in FIG. 31.

Switching elements M1, M2 respectfully connect between the cathodes 434of the indicator LEDs 402, 404 and the second voltage reference 438. Theswitching elements M1, M2 control current flow through the indicatorLEDs 402, 404 thereby controlling operation of the indicator LEDs 402,404. The switching elements M1, M2 may be further defined astransistors, and more specifically, N-channel MOSFETs. Each of theMOSFETs M1, M2 includes a gate, source, and drain. The gate of eachMOSFET M1, M2 connects to the tracker controller 62. The source of eachMOSFET M1, M2 connects to the second reference voltage 438, and morespecifically, signal ground. The drain of each MOSFET M1, M2 connects tothe cathode 434 of the respective indicator LED 402, 404. Resistors R1,R2 respectfully connect between the cathode 434 of each of the indicatorLEDs 402, 404 and the drain of each MOSFET M1, M2. The resistors R1, R2limit current flow through the indicator LEDs 402, 404 to suitableoperating levels.

The tracker controller 62 controls activation/deactivation of theindicator LEDs 402, 404. The tracker controller 62 selectively controlsthe switching elements M1, M2 to allow or prevent current flow throughthe indicator LEDs 402, 404.

In one embodiment, the tracker controller 62 sends a first indicatorcontrol signal to the gate of the MOSFET M1. The tracker controller 62may send the first indicator control signal in response to the trackercontroller 62 receiving the error signal from the navigation computer26. The first indicator control signal causes the MOSFET M1 to form aclosed circuit path between the source and the drain such that currentfreely flows through indicator LED 402 between the first and secondvoltage references 436, 438 to illuminate indicator LED 402.

In other embodiments, the tracker controller 62 sends a second indicatorcontrol signal to the gate of the MOSFET M2. The tracker controller 62may send the second indicator control signal in response to the trackercontroller 62 receiving an all clear signal or no error signal from thenavigation computer 26. In turn, the second indicator control signalcauses the MOSFET M2 to form a closed circuit path between the sourceand the drain such that current freely flows through indicator LED 404between the first and second voltage references 436, 438 to illuminateindicator LED 404.

The first and second indicator control signals may correspond to anysuitable predetermined voltage or current for controlling MOSFETs M1,M2.

The tracker controller 62 may alternate activation/deactivation of theindicator LEDs 402, 404. In one embodiment, the tracker controller 62sends the second indicator control signal to the gate of the MOSFET M2to deactivate indicator LED 404. According to one embodiment, thetracker controller 62 does so during activation of indicator LED 402. Todeactivate indicator LED 404, the second indicator control signal causesthe MOSFET M2 to form an open circuit between the source and the drainsuch that current is prevented from flowing through indicator LED 404between the first and second voltage references 436, 438. Alternatively,the tracker controller 62 may send the first indicator control signal tothe gate of the MOSFET M1 to deactivate indicator LED 402 duringactivation of LED indicator 404.

In one embodiment, the indicator LEDs 402, 404 may be selectivelydetachable from and attachable to the tracking head 412. The trackinghead 412 may include a printed circuit board (PCB) assembly (not shown)disposed therein with the indicator LEDs 402, 404 being electricallyconnected to the PCB assembly. In FIG. 31, the indicator LEDs 402, 404are coupled to a unit 440 that is detachable from and attachable to thePCB assembly. For simplicity, the unit 440 is represented in FIG. 31 bya dotted line surrounding indicator LEDs 402, 404. The unit 440 includesa first, second, and third terminal 442, 444, 446 for selectivelyconnecting the indicator LEDs 402, 404 to the PCB assembly. The firstterminal 442 selectively connects the anode 432 of each of the indicatorLEDs 402, 404 to the first voltage reference 436. The second and thirdterminals 444, 446 selectively connect the cathode 434 of each of theindicator LEDs 402, 404 to the MOSFETs M1, M2 for each respectiveindicator LED 402, 404.

Control of the LEDs 50 is carried out using the circuit shown in FIG.32. FIG. 32 shows an electrical schematic of the LEDs 50 connected tothe tracker controller 62. The electrical schematic shows the electricalcomponents that facilitate control of the LEDs 50.

In FIG. 32, each of the LEDs 50 includes an anode 450 and a cathode 452.The anode 450 of each of the LEDs 50 connects to a third voltagereference 454 while the cathode 452 of each of the LEDs 50 connects to afourth voltage reference 456. In one embodiment, the third voltagereference 454 is +2 VDC and the fourth voltage reference 456 is signalground. The anode 450 of each of the LEDs 50 may connect to the thirdvoltage reference 454 separately, or in combination, as shown in FIG.32.

Switching elements M3, M4, M5, M6 respectively connect to the cathodes452 of the LEDs 50. The switching elements M3, M4, M5, M6 controlcurrent flow through the LEDs 50 thereby controlling operation of theLEDs 50. The switching elements M3, M4, M5, M6 may be further defined astransistors, and more specifically, N-channel MOSFETs. Each of theMOSFETs M3, M4, M5, M6 includes a gate, source, and drain. The gate ofeach MOSFET M3, M4, M5, M6 connects to the tracker controller 62. Thesource of each MOSFET M3, M4, M5, M6 ultimately connects to the fourthvoltage reference 456, and more specifically, signal ground. In FIG. 32,the sources of the MOSFETs M3, M4, M5, M6 are connected to a shared line458 in a parallel configuration. The drain of each MOSFET M3, M4, M5, M6respectively connects to the cathode 452 of one of the LEDs 50.

The tracker controller 62 controls activation/deactivation of the LEDs50. Mainly, the tracker controller 62 selectively controls the switchingelements M3, M4, M5, M6 to allow or prevent current flow through theLEDs 50.

In one embodiment, the tracker controller 62 receives an input signalindicating how the tracker controller 62 is to control any given LED 50or combination of LEDs 50. The tracker controller 62 may receive theinput signal from the navigation computer 26. In response, the trackercontroller 62 sends an LED control signal to the gate of the MOSFET orMOSFETs M3, M4, M5, M6 connected to any given LED 50 or combination ofLEDs 50. In FIG. 32, the tracker controller 62 sends four separate LEDcontrol signals and each LED control signal is sent to the gate of eachMOSFET M3, M4, M5, M6. In one embodiment, the tracker controller 62sequentially sends the control signals to the MOSFETs M3, M4, M5, M6 tosequentially control activation or deactivation of the LEDs 50. Thetracker controller 62 may send the LED control signals to the MOSFETsM3, M4, M5, M6 for controlling the LEDs 50 according various otherconfigurations, sequences, cycles, or combinations.

If the input signal indicates that the tracker controller 62 is toactivate any given LED 50, the tracker controller 62 sends the LEDcontrol signal to the gate of the respective MOSFET M3, M4, M5, M6.Doing so causes the MOSFET M3, M4, M5, M6 to form a closed circuit pathbetween the source and the drain of such that current freely flowsthrough the respective LED 50 between the third and fourth voltagereferences 454, 456 to illuminate the respective LED 50.

If no input signal is received by the tracker controller 62 or when theinput signal indicates that the tracker controller 62 is to deactivateany given LED 50, the tracker controller 62 sends the LED control signalto the gate of the MOSFET M3, M4, M5, M6 to deactive the given LED 50.Mainly, the LED control signal causes the MOSFET M3, M4, M5, M6 to forman open circuit between the source and the drain of the MOSFET such thatcurrent is prevented from flowing between the third voltage reference454 and signal ground 456 through the LED 50.

The LEDs 50 may be detachable from and attachable to the tracking head412. In FIG. 32, each LED 50 is coupled to a unit 460 that is detachablefrom and attachable to the PCB assembly. For simplicity, each unit 460is represented in FIG. 32 by a dotted line surrounding each of the LEDs50. In one embodiment, each unit 460 includes a first and a secondterminal 462, 464 for selectively connecting each LED 50 to the PCBassembly. In FIG. 32, the first terminal 462 selectively connects theanode 450 of each LED 50 to the third voltage reference 454. The secondterminal 464 selectively connects the cathode 452 of each LED 50 todrain of each respective MOSFET M3, M4, M5, M6.

In FIG. 32, a voltage sensing circuit 470 is provided for measuringoperating voltages of any given LED 50 or combination of LEDs 50. Thevoltage sensing circuit includes a resistor R3 and a capacitor C1 thatare arranged as a series RC circuit. The voltage sensing circuit 470 isgenerally connected between the LEDs 50 and the tracker controller 62.Resistor R3 connects to the sources of the MOSFETs M3, M4, M5, M6 at theshared line 458 and capacitor C1 connects between resistor R3 and thefourth reference voltage 456. In one embodiment, the voltage sensingcircuit 470 measures operating voltage of the LEDs 50 between the thirdand fourth voltage reference 454, 456.

The voltage sensing circuit 470 sends to the tracker controller 62 avoltage sense signal representing a measured operating voltage of theLEDs 50. In one embodiment, the voltage sensing circuit 470 protects theLEDs 50 from inappropriate voltage conditions. The tracker controller 62may process the voltage sense signal and, in response, modify the LEDcontrol signal based on the value of the voltage sense signal. Forinstance, the tracker controller 62 may change the voltage of the LEDcontrol signal(s) if the tracker controller 62 determines that thevoltage sense signal is above a predetermined threshold level. Inanother embodiment, the tracker controller 62 utilizes the voltagesensing circuit 470 for determining whether an LED 50 is malfunctioning.The tracker controller 62 can communicate the malfunction of the LED 50to the navigation system 20 so that the navigation system 20 cananticipate such malfunction and respond accordingly. The voltage sensingcircuit 470 may be implemented according to various other configurationsand methods.

In FIG. 32, a current sensing circuit 480 is provided for measuringoperating currents of any given LED 50 or combination of LEDs 50. Thecurrent sensing circuit 480 protects the LEDs 50 from inappropriatecurrent conditions. The current sensing circuit 480 includes a resistorR4 and a capacitor C2 that are arranged as a series RC circuit. Thecurrent sensing circuit 480 is connected generally between the LEDs 50and the tracker controller 62. Resistor R4 connects to the sources ofthe MOSFETs M3, M4, M5, M6 at the shared line 458 and capacitor C2connects between resistor R4 and signal ground 456. In one embodiment,the current sensing circuit 480 measures total operating current of theLEDs passing between the third and fourth voltage reference 454, 456.

In one mode of operation, the current sensing circuit 480 provides tothe tracker controller 62 a current sense signal. The current sensesignal may be derived from the measured operating current of the LEDs50. The tracker controller 62 may process the current sense signal anddetermine whether the current sense signal conforms to a predeterminedvalue. For instance, the tracker controller 62 may determine that thecurrent sense signal is above a predetermined threshold voltage level.

A current limiting circuit 482 is further provided in FIG. 32 forlimiting current provided to the LEDs 50. In FIG. 32, the currentlimiting circuit 482 includes an amplifier 484 for regulating thecurrent through the LEDS 50. The amplifier 484 includes a first andsecond input terminal 486, 488 and an output terminal 490. The currentlimiting circuit 482 includes a MOSFET M7 for controlling the currentthrough the LEDs 50. The MOSFET M7 includes a gate, a source, and adrain.

The first input terminal 486 of the amplifier 482 connects to thetracker controller 62. The second input terminal 488 of the amplifier482 connects to the gate and the source of the MOSFET M7. A capacitor C3is included between the second input terminal 488 and the gate of theMOSFET M7. A resistor R5 is included between the second input terminal488 and the source of the MOSFET M7. Resistor R6 connects between thesource of the MOSFET M7 and the signal ground 456. The output terminal490 of the amplifier 482 connects to the gate of the MOSFET M7. ResistorR7 connects between the output 490 of the amplifier 484 and the gate ofthe MOSFET M7. The drain of the MOSFET M7 connects to the sources of theMOSFETs M3, M4, M5, M6 at the shared line 458.

In one mode of operation, the tracker controller 62 sends a currentlimiting signal to the current limiting circuit 482. The trackercontroller 62 may send the current limiting signal based on the value ofthe current sense signal provided by the current sensing circuit 480. InFIG. 32, the current limiting signal sent by the tracker controller 62is received at the first input terminal 486 of the amplifier 482. Theamplifier 484 controls the MOSFET M7 based on the current limitingsignal received from the tracker controller 62. In one instance, theamplifier 484 deactivates each of the LEDs 50 in response to the currentlimiting signal. Specifically, the amplifier 484 delivers a signal fromthe output terminal 490 to the gate of MOSFET M7. Doing so causes MOSFETM7 to form an open circuit between the source and the drain. As such,current is prevented from flowing between the third and fourth voltagereference 454, 456 thereby deactivating each of the LEDs 50. In otherembodiments, the amplifier limits current through the LEDs 50, but doesnot entirely deactivate the LEDs 50, in response to the current limitingsignal.

The current sensing circuit 480 and the current limiting circuit 482 maybe implemented according to various other configurations and methods.

In some embodiments, the tracker 44 may include four or more trackingLEDs 50 so that if the tracking signal from one of the LEDs 50 isobstructed, position and orientation data can still be obtained from theremaining LEDs 50. In this instance, before any error signals aregenerated, the navigation computer 26 will first run through a completetracking cycle. The complete tracking cycle includes sequentiallyactivating all the LEDs 50 on the tracker 44 to determine if the opticalsensors 40 receive tracking signals from at least three of the LEDs 50in the tracking cycle. The error signal is then generated if an opticalsensor 40 (or all optical sensors 40 in some embodiments) did notreceive tracking signals from at least three LEDs 50 in the trackingcycle.

The navigation system 20 is configured to assist with positioning of thetracker 44 by the surgeon or other medical personnel. This assistancehelps to place the tracking head 212 (or tracking heads 312, 412) in adesired orientation that provides line-of-sight between the LEDs 50 andthe optical sensors 40 and helps to reduce line-of-sight errors that mayotherwise be encountered during a surgical procedure.

Once the bone plate 200 is mounted to the bone, such as femur F, thetracking head 212 is movable relative to the bone plate 200 via theconnector assembly 214. In particular, the tracking head 212 is movableabout pivot axis P and rotational axis R (see FIG. 4). The connectorassembly 214 allows movement of the tracking head 212 in these twodegrees of freedom relative to the bone plate 200 to place the trackinghead 212 in the desired orientation. This helps to provide theline-of-sight between the LEDs 50 and the optical sensors 40.

Before navigation begins, the medical personnel are instructed to placethe tracking head 212 in an initial orientation in which, visually, thetracking head 212 appears to be oriented so that the LEDs 50 will bewithin the line-of-sight of the optical sensors 40 and unobstructedthroughout the surgical procedure. Once the user has placed the trackinghead 212 in the initial orientation, the navigation computer 26 providesinstructions to the medical personnel setting up the tracker 44 on howto further move the tracking head 212 to reach the desired orientation,if necessary.

The navigation computer 26 first determines the initial orientation ofthe tracking head 212 and whether the tracking head 212 is already inthe desired orientation. If the tracking head 212 is not in the desiredorientation, the navigation system 20, through software instructionsdisplayed on displays 28, 29, instructs the user to move the trackinghead 212 relative to the bone plate 200 in one or more of the degrees offreedom to reach the desired orientation.

The tracker 44 includes an orientation sensor 320, as shown in FIG. 22.In one embodiment, the orientation sensor 320 is a three-axis gravitysensor 320 configured to measure gravity (acceleration) along threeaxes, the x-axis, y-axis, and z-axis.

The gravity sensor 320 is located inside the tracking head 212. Thegravity sensor 320 is operatively connected to the tracker controller62. The tracker controller 62 receives gravity measurements from thegravity sensor 320 with respect to the x-axis, y-axis, and z-axis. Thesesignals are analyzed by the navigation system 20 to determine thecurrent orientation of the tracking head 212 relative to gravity. Itshould be appreciated that the accelerometer 70, in some embodiments,could be used as the gravity sensor 320.

Referring to FIG. 22, the x-axis and y-axis are oriented in a plane ofthe tracking head 212 that is parallel with a front face of the trackinghead 212. The z-axis is oriented perpendicular to the x-axis and y-axis.When the gravity measurement along the z-axis is zero, then gravity isnot acting along the z-axis meaning that the x-y plane of the trackinghead 212 is oriented vertically.

Referring to FIGS. 33A and 33B, in one embodiment, the desiredorientation includes having the tilt angle about pivot axis P and therotational axis R set to angles resulting in the z-axis gravitymeasurement being zero.

Referring to FIG. 33A, the signals transmitted by the LEDs 50 areconsidered line-of-sight tracking signals that have a central signalaxis 322 defining the center of a signal emitting region 324. Thedesired orientation is further defined in one embodiment as the centralsignal axes 322 of the LEDs 50 being perpendicular to the direction ofgravity, e.g., perpendicular to a vector 326 oriented in the directionof gravity.

In this embodiment, the navigation computer 26 is configured to instructthe user to adjust the tilt angle of the tracking head 212 until thez-axis gravity measurement is zero. This occurs when the x-y plane ofthe tracking head 212 is oriented vertically relative to the gravityvector 326, as shown in FIG. 33B.

The instructions to the user to adjust the tilt angle are carried out bythe navigation system 20 through displays 28, 29, in which the currentorientation of the tracking head 212 is graphically represented. Thecurrent orientation is dynamically adjusted as the user changes the tiltangle. The desired orientation of the tracking head 212 is also showngraphically so that the user can visually determine how close thecurrent orientation is to the desired orientation. When the currentorientation is at the desired orientation, the displays 28, 29 may flashgreen or have some other visual indicator that the tracking head 212 isin the desired orientation relative to gravity. It should be appreciatedthat the desired orientation may include predefined deviations from anideal orientation in which the x-y plane is perfectly vertical relativeto gravity, such as deviations of +/− ten percent, +/− five percent, or+/− two percent.

In alternative embodiments, an LED on the tracking head 212 may indicateto the user when the current orientation is at the desired orientationby being activated to emit a green colored light. Alternatively, anaudible indicator may be provided on the tracker 44 to indicate that thetracking head 212 is in the desired orientation.

Once the tilt angle is set so that the tracking head 212 is at thedesired orientation relative to gravity (see, e.g., FIG. 33B), theadjustment fastener 286 of the connector assembly 214 is tightened sothat the tracking head 212 is unable to tilt relative to the bone plate200. In some cases only tilt adjustment of the tracking head 212 may benecessary to place the tracking head 212 in the desired orientation.

In other cases, rotational adjustment about rotational axis R may beneeded after the tilt adjustment to place the tracking head 212 in thedesired orientation. In these cases, the desired orientation may includean additional rotational adjustment in which the gravity measurementalong the z-axis moves from being approximately zero to being non-zero.Adjustment may also be iterative in which tilt adjustment is performedfirst to place the tracking head 212 vertically, then rotationaladjustment is performed which causes the gravity measurement along thez-axis to be non-zero, and then further tilt adjustment is performed toplace the z-axis back to approximately zero (i.e., to move the trackinghead 212 back to vertical).

During rotational adjustment, the LEDs 50 are being tracked by theoptical sensors 40. In particular, the navigation computer 26 isconfigured to determine, based on the signals received by the opticalsensors 40, which rotational orientation of the tracking head 212provides the best line-of-sight to the LEDs 50.

The desired rotational orientation can be determined by instructing theuser to rotate the tracking head 212 through a maximum range ofmovement, e.g., 360 degrees, one or more times. While the tracking head212 is rotated, the navigation computer 26 determines at whichrotational positions (e.g., rotational angles) about axis R line ofsight for each LED 50 is present and at which rotational positions thereis no line of sight for each LED 50. This may include rotating thetracking head 212 though its maximum range of movement at variouspositions of the knee joint, i.e., at the flexed and at the extendedpositions of the knee joint. This will determine a range ofline-of-sight positions about axis R for each LED 50. A best fitalgorithm can then be used to determine the positions about axis R thatbest fits within the ranges of line-of-sight positions for all of theLEDs 50.

The navigation computer 26 instructs the user, through the displays 28,29 to rotate the tracking head 212 until the current rotational positionmeets the desired rotational position determined by the navigationcomputer 26. See, for instance, the desired rotational position shown inFIG. 34B, as compared to a current rotational position shown in FIG.34A. Instruction to the user may be accomplished by showing arepresentation of the current rotational position and the desiredrotational position on the displays 28, 29 and dynamically changing thecurrent rotational position as the user makes the adjustment toward thedesired rotational position.

Like with adjusting the tilt angle, an LED on the tracking head 212 mayindicate to the user when the rotational position is at the desiredrotational position by being activated to emit a green colored light.Alternatively, an audible indicator may be provided on the tracker 44 toindicate that the current rotational position is at the desiredrotational position. It should be appreciated that the desiredrotational position may include predefined deviations from an idealrotational position, such as deviations of +/− ten percent, +/− fivepercent, or +/− two percent.

When the desired rotational orientation is met, the adjustment fastener261 of the connector assembly 214 is tightened so that the tracking head212 is unable to rotate relative to the bone plate 200. The trackinghead 212 is now fixed from moving relative to the bone plate 200 and thebone.

In some embodiments, the LEDs 50 are being tracked by the opticalsensors 40 during adjustment of both the tilt angle and rotationalangle. In particular, the navigation computer 26 is configured todetermine, based on the signals received by the optical sensors 40,which tilt and rotational orientation of the tracking head 212 providesthe best line-of-sight from the LEDs 50 to the optical sensors 40.

In these embodiments, the desired orientation can be determined byinstructing the user to tilt and rotate the tracking head 212 throughtheir maximum ranges of movement, one or more times, either sequentiallyor alternately. While the tracking head 212 is tilted and rotated, thenavigation computer 26 determines at which tilt and rotational positions(e.g., tilt and rotational angles) about axes P and R line of sight foreach LED 50 is present and at which tilt and rotational positions thereis no line of sight for each LED 50. This will determine a range ofline-of-sight positions about axes P and R for each LED 50. A best fitalgorithm can then be used to determine the positions about axes P and Rthat best fits within the ranges of line-of-sight positions for all ofthe LEDs 50.

This process may be iterative and include several adjustments by theuser about the axes P and R to find a suitable position for the trackinghead 212. In certain instances, the navigation computer 26 may be unableto identify an orientation in which line-of-sight is maintained for allof the LEDs 50 because of a poor initial orientation set by the user. Inthis case, the navigation computer 26 may first instruct the user toreorient the tracking head 212 so that the LEDs 50 visually appear to befacing the optical sensors 40 and then continue with measuring theorientation of the tracking head 212 through various movements to findthe best fit that maintains the line-of-sight for all of the LEDs 50.

In some cases, tilting adjustment may be processed first with thetracking head 212 being moved through its entire range of tiltingmotion, one or more times, and possibly at multiple knee positionsincluding flexed and extended positions. The best fit tilt angle is thendetermined and the tilt angle is then fixed at the best fit tilt angle.The tracking head 212 can thereafter be moved through its entire rangeof rotational motion, one or more times, and possibly at multiple kneepositions including flexed and extended positions. The best fitrotational angle is then determined and the rotational angle is thenfixed at the best fit rotational angle.

In some embodiments, a third degree of freedom may be adjusted to adesired position, such as a height of the tracker 44. In the embodimentshown only two degrees of freedom are adjusted due to the fact that thetracker 44 moves up and down as the surgeon flexes the knee joint. Inthis case, the LEDs 50 of the tracking head 212 may be raised or loweredrelative to the optical sensors 40 without breaking the line-of-sightbetween the LEDs 50 and sensors 40.

A screw driver 500 is shown in FIGS. 35-51. The screw driver 500 is usedto place the bone screws 202.

Referring to FIG. 36, the screw driver 500 includes an integratedimpactor 502 that punches each bone screw 202 into the bone prior toscrewing. When the screw driver 500 is pressed against the bone screw202, the impactor 502 stores energy in a rear spring 504. The energy iseventually released as a force that drives the bone screw 202 into thebone. The impactor 502 starts in a rest state in which no energy isstored, is operated to gradually increase the stored energy, and then isactuated to release the energy.

The screw driver 500 includes a body. The body comprises a nose tube508, middle tube 510, and rear cap 512. The nose tube 508, middle tube510, and rear cap 512 are separate parts that are releasably connectedtogether for purposes of assembling internal components. It should beappreciated that in other embodiments, the nose tube 508, middle tube510, and rear cap 512 may be permanently fixed together.

The nose tube 508 has a generally conical distal end 514. The nose tube508 extends from its distal end 514 to an externally threaded proximalend 516. The nose tube 508 is hollow. The nose tube 508 defines aproximal bore 518 and distal bore 520. The proximal bore 518 is largerin cross-sectional area than the distal bore 520. The proximal bore 518is circular in cross-section and the distal bore 520 is hexagonal incross-section.

The middle tube 510 is generally cylindrical. The middle tube 510 has aninternally threaded distal end 522 and an externally threaded proximalend 524. The internally threaded distal end 522 threads onto theexternally threaded proximal end 516 of the nose tube 508.

The rear cap 512 has a top 526. The rear cap 512 extends distally fromthe top 526 to an internally threaded section 528. The internallythreaded section 528 threads onto the externally threaded proximal end524 of the middle tube 510.

The impactor 502 includes a hammer 530 disposed in the middle tube 510.The hammer 530 is generally cylindrical in shape. The rear spring 504 isdisposed between the rear cap 512 and the hammer 530. The rear spring504 biases the hammer 530 distally. Compression of the rear spring 504can be adjusted by loosening or tightening the rear cap 512 to decreaseor increase the stored energy of the hammer 530.

A receiving hole 532 is formed in the hammer 530. The receiving hole 532is disposed about a central axis of the hammer 530. The receiving hole532 is cylindrically shaped. A cross bore 534 is formed in a directionperpendicular to the receiving hole 532 (see FIG. 46).

The impactor 502 includes a trigger 536 located in the cross bore 534.The trigger 536 is semi-cylindrical in shape. The trigger 536 has a flatbottom 538 (see FIG. 47). A throughbore 540 is defined in the trigger536. When the impactor 502 is in the rest state, a leaf spring 542biases the trigger 536 to a position in which the throughbore 540 isoffset to the receiving hole 532, i.e., they are misaligned.

A driving rod 544 ultimately receives the energy stored in the impactor502 to drive in the bone screws 202. The driving rod 544 has acylindrical shaft 546 with a proximal end 548. The proximal end 548 isshaped for mating reception within the receiving hole 532 of the hammer530.

A boss 550 is located on the proximal end 548 of the driving rod 544 toform a shoulder 552 (see FIG. 50). In the rest state of the impactor502, the shoulder 552 engages the flat bottom 538 of the trigger 536. Inthis state, the boss 550 protrudes into the throughbore 540 defined inthe trigger 536, but the shoulder 552 prevents the cylindrical shaft 546from entering the throughbore 540.

The driving rod 544 has a hexagonal shaft 554 with a distal end 556. Thehexagonal shaft 554 mates with the hexagonal-shaped distal bore 520 ofthe nose tube 508 so that rotation of the body by the user also rotatesthe hexagonal shaft 554. The distal end 556 has features adapted toengage heads of the bone screws 202 to rotate and drive the bone screws202 into the bone when the body of the screw driver 500 is rotated.

A collar 558 is fixed to the hexagonal shaft 554. The collar 558prevents the driving rod 544 from falling out of the nose tube 508. Thecollar 558 is cylindrical in shape to mate with the proximal bore 518 ofthe nose tube 508.

A spring cap 560 is centrally disposed inside the middle tube 510between the nose tube 508 and the hammer 530. The spring cap 560 ispress fit inside the middle tube 510.

A rod spring 562 is disposed about the cylindrical shaft 546 of thedriving rod 544. The rod spring 562 acts between the spring cap 560 andthe collar 558 of the driving rod 544 to return the screw driver 500 tothe rest state after the hammer 530 is actuated. The spring cap 560defines a throughbore (not numbered) for receiving the cylindrical shaft546 of the driving rod 544 and centering the cylindrical shaft 546inside the middle tube 510.

The screw driver 500 is pressed against the bone screw 202 by grippingthe rear cap 512 and/or middle tube 510 and urging them distally. Thebone screw 202 is thus pressed against the bone. At the same time, thedriving rod 544 travels proximally in the middle tube 510. The shoulder552 pushes the trigger 536 proximally while the leaf spring 542 keepsthe trigger 536 throughbore 540 misaligned with the receiving hole 532of the hammer 530.

The middle tube 510 includes an inclined inner surface 566. The inclinedinner surface 566 engages the trigger 536 when the trigger 536 reachesthe inclined inner surface 566. When this occurs, the inclined innersurface 566 acts like a cam to push the trigger 536 in a manner thatcenters the throughbore 540 of the trigger 536 and places thethroughbore 540 into alignment with the receiving hole 532 of the hammer530. Likewise, the proximal end 548 of the driving rod 544 is nowaligned to fit within the throughbore 540, which allows the trigger 536to slide down the driving rod 544 thereby releasing the hammer 530. Thehammer 530 then moves forward, propelled by the rear spring 504.

Because the receiving hole 532 in the hammer 530 has a predefined depth,the boss 550 on the proximal end 548 of the driving rod 544 eventuallybottoms out in the receiving hole 532 and the force of the hammer 530 istransmitted into the driving rod 544 and the bone screw 202 punching thebone screw 202 into the bone.

OTHER EMBODIMENTS

In one embodiment, when each of the trackers 44, 46, 48 are beingactively tracked, the firing of the LEDs occurs such that one LED 50from tracker 44 is fired, then one LED 50 from tracker 46, then one LED50 from tracker 48, then a second LED 50 from tracker 44, then a secondLED 50 from tracker 46, and so on until all LEDs 50 have been fired andthen the sequence repeats. This order of firing may occur throughinstruction signals sent from the transceivers (not shown) on the cameraunit 36 to transceivers (not shown) on the trackers 44, 46, 48 orthrough wired connections from the navigation computer 26 to the trackercontroller 62 on each of the trackers 44, 46, 48.

The navigation system 20 can be used in a closed loop manner to controlsurgical procedures carried out by surgical cutting instruments. Boththe instrument 22 and the anatomy being cut are outfitted with trackers50 such that the navigation system 20 can track the position andorientation of the instrument 22 and the anatomy being cut, such asbone.

In one embodiment, the navigation system 20 is part of a roboticsurgical system for treating tissue. In some versions, the roboticsurgical system is a robotic surgical cutting system for cutting awaymaterial from a patient's anatomy, such as bone or soft tissue. Thecutting system could be used to prepare bone for surgical implants suchas hip and knee implants, including unicompartmental, bicompartmental,or total knee implants. Some of these types of implants are shown inU.S. patent application Ser. No. 13/530,527, entitled, “ProstheticImplant and Method of Implantation”, the disclosure of which is herebyincorporated by reference.

In one embodiment, the navigation system 20 communicates with a roboticcontrol system (which can include the manipulator controller 54). Thenavigation system 20 communicates position and/or orientation data tothe robotic control system. The position and/or orientation data isindicative of a position and/or orientation of instrument 22 relative tothe anatomy. This communication provides closed loop control to controlcutting of the anatomy such that the cutting occurs within a predefinedboundary.

In one embodiment, each of the femur F and tibia T has a target volumeof material that is to be removed by the working end of the surgicalinstrument 22. The target volumes are defined by one or more boundaries.The boundaries define the surfaces of the bone that should remain afterthe procedure. In some embodiments, navigation system 20 tracks andcontrols the surgical instrument 22 to ensure that the working end,e.g., bur, only removes the target volume of material and does notextend beyond the boundary, as disclosed in U.S. patent application Ser.No. 13/958,070, filed Aug. 2, 2013, entitled, “Surgical ManipulatorCapable of Controlling a Surgical Instrument in Multiple Modes”, thedisclosure of which is hereby incorporated by reference.

Several embodiments have been discussed in the foregoing description.However, the embodiments discussed herein are not intended to beexhaustive or limit the invention to any particular form. Theterminology which has been used is intended to be in the nature of wordsof description rather than of limitation. Many modifications andvariations are possible in light of the above teachings and theinvention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A navigation system for tracking a plurality ofobjects, said navigation system comprising: a sensor; a plurality oftracking devices attachable to the plurality of objects, with each ofsaid tracking devices including: a tracking head having a tracking bodyand a tracking element that is coupled to said tracking body andconfigured to transmit a tracking signal to said sensor to determine aposition of one of the plurality of objects; and an error indicatorsupported by said tracking head and configured to indicate an error insaid sensor receiving said tracking signal from said tracking element,said error indicator including a first light emitting diode attached tosaid tracking body and a second light emitting diode attached to saidtracking body; a computing system configured to: determine if saidsensor received said tracking signal from said tracking element,generate an error signal if said sensor did not receive said trackingsignal, and control activation of said first and second light emittingdiodes of said error indicator plurality of error indicator to emit afirst colored light in response to generating said error signal and toemit a second colored light in the absence of said error signal.
 2. Anavigation system as set forth in claim 1 wherein said computing systemis configured to activate said first light emitting diode to emit saidfirst colored light in response to generating said error signal anddeactivate said second light emitting diode to discontinue emitting saidsecond colored light in response to generating said error signal.
 3. Anavigation system as set forth in claim 1 wherein said computing systemis configured to activate said first and second light emitting diodes toemit said first colored light in response to generating said errorsignal and to emit said second colored light in the absence of saiderror signal.
 4. A navigation system as set forth in claim 1 whereinsaid tracking body includes a top, bottom, and sides, said tracking bodydefining a light opening in said sides to receive said first coloredlight and said second colored light from said first and second lightemitting diodes.
 5. A navigation system as set forth in claim 1 whereinsaid sensor has a field of view and each one of said plurality oftracking devices is attachable to a corresponding one of the pluralityof objects so that said tracking element is in said field of view.
 6. Anavigation system as set forth in claim 1 wherein said tracking head ofeach one of said plurality of tracking devices further comprises asecond tracking element.
 7. A navigation system as set forth in claim 6wherein said tracking elements are line-of-sight tracking elements.
 8. Anavigation system as set forth in claim 6 wherein said tracking elementsare light emitting diodes.
 9. A navigation system as set forth in claim6 wherein said tracking elements are reflector balls.
 10. A navigationsystem as set forth in claim 1 wherein said tracking head of each one ofsaid plurality of tracking devices further comprises second and thirdtracking elements for transmitting tracking signals.
 11. A navigationsystem as set forth in claim 10 wherein said computing system isconfigured to: determine if said sensor receives said tracking signalsfrom all three of said tracking elements of said tracking head in atracking cycle that includes sequential activation of all three of saidtracking elements; and generate said error signal if said sensor did notreceive said tracking signals from all three of said tracking elementsof said tracking head in said tracking cycle.
 12. A navigation systemfor tracking a plurality of objects, said navigation system comprising:a computer cart assembly comprising a computer and an optical positionsensor communicating with one another; and a plurality of trackingdevices attachable to the plurality of objects, with each of saidtracking devices including; a tracking head having a tracking body and atracking element that is coupled to said tracking body and configured totransmit a tracking signal to said optical position sensor fordetermining a position of one of the plurality of objects; and an errorindicator supported by said tracking head and configured to indicate anerror in said sensor receiving said tracking signal from said trackingelement, said error indicator including a first light emitting diodeattached to said tracking body and a second light emitting diodeattached to said tracking body; wherein said computer is configured to:determine if said optical position sensor received said tracking signalfrom said tracking element, generate an error signal if said sensor didnot receive said tracking signal from said tracking element, and controlactivation of said light emitters first and second light emitting diodesof said error indicator to emit a first colored light in response togenerating said error signal and to emit a second colored light in theabsence of said error signal.
 13. A navigation system as set forth inclaim 12 further wherein said computer cart assembly includes an arm anda camera unit coupled to said arm, with said camera unit including saidoptical position sensor.
 14. A method for tracking a plurality ofobjects using a navigation system comprising a sensor and a plurality oftracking devices, said method comprising the steps of: providing theplurality of tracking devices attachable to the plurality of objects,with each of the tracking devices including: a tracking head having atracking body and a tracking element that is coupled to the trackingbody and configured to transmit a tracking signal to the sensor todetermine a position of one of the plurality of objects; and an errorindicator supported by the tracking head and configured to indicate anerror in the sensor receiving the tracking signal from the trackingelement, the error indicator including a first light emitting diodeattached to the tracking body and a second light emitting diode attachedto the tracking body; transmitting the tracking signal from the trackingelement to the sensor while the tracking device is attached to theobject; determining if the sensor received the tracking signal from thetracking element; generating an error signal if the sensor did notreceive the tracking signal; and controlling activation of the first andsecond light emitting diodes supported by the tracking head to emit afirst colored light in response to generating the error signal and toemit a second colored light in the absence of the error signal.
 15. Amethod as set forth in claim 14 wherein controlling activation of thefirst and second light emitting diodes includes activating the firstlight emitting diode to emit the first colored light in response togenerating the error signal and deactivating the second light emittingdiode to discontinue emitting the second colored light in response togenerating the error signal.
 16. A method as set forth in claim 14wherein controlling activation of the first and second light emittingdiodes includes activating the first and second light emitting diodes toemit the first colored light in response to generating the error signaland to emit the second colored light in the absence of the error signal.17. A method as set forth in claim 14 wherein controlling activation ofthe first and second light emitting diodes to emit the first coloredlight and the second colored light includes emitting the first coloredlight and the second colored light through a light opening defined insides of a tracking body.
 18. A method as set forth in claim 14 whereinthe tracking device is attached to the object so that the trackingelement is within a field of view of the sensor.
 19. A method as setforth in claim 14 wherein the tracking head has second and thirdtracking elements and said method further comprises transmittingtracking signals from the second and third tracking elements.
 20. Amethod as set forth in claim 19 including determining if the sensorreceives the tracking signals from all three of the tracking elements ofthe tracking head in a tracking cycle that includes sequentialactivation of all three of the tracking elements and generating theerror signal if the sensor did not receive the tracking signals from allthree of the tracking elements of the tracking head in the trackingcycle.